1. Field of the Invention
The present invention relates to a halogen-free flame retardant thermoplastic polyurethane composite resin composition. More particularly, the present invention relates to a flame retardant thermoplastic polyurethane composite resin composition capable of improving flame retardancy and flaming-drip of a thermoplastic polyurethane resin through desirable formation of char during combustion, which uses a phosphinate, a diphosphinate and/or a polymer thereof, dipentaerythritol, talc, a melamine derivative, etc., instead of a halogen-based flame retardant, as a flame retardant.
2. Description of the Prior Art
In general, a thermoplastic polyurethane resin, which has an excellent mechanical property (such as high abrasion resistance) and a high elastic force, can be used for producing products through processes such as injection-molding, extrusion-molding, etc., unlike a conventional thermosetting resin (that is, an elastomer such as cross-linked rubber), and also due to its excellent formability, has been used in various industrial fields, such as cars, electric wires, pneumatic hoses, shoes, etc. However, a thermoplastic polyurethane resin has weak flame resistance, and thereby its use has been restricted in certain fields requiring high flame resistance. Accordingly, methods of providing flame resistance to such a thermoplastic polyurethane resin have been developed, and especially, some methods of adding a flame resistant to a resin have been mainly used. Since the addition of a flame retardant may reduce physical properties of a thermoplastic polyurethane resin, such as elongation at break, a resilient elastic force, elastic modulus, abrasion, etc., it is preferable to add the flame retardant to the resin in as small an amount as possible so as to minimize the reduction. Also, the thermoplastic polyurethane resin is decomposed into a low molecular weight molten material through combustion, thereby causing flaming-drip. In this case, if a fire occurs, the flaming-drip may spread the fire. Therefore, improvement of the occurrence of flaming-drip during combustion is one of many important things to be taken into account in developing a thermoplastic polyurethane resin.
One method of improving flame retardancy of a thermoplastic polyurethane resin is to use a halogen-based flame retardant either alone or in combination with a metal oxide compound, such as antimony oxide, etc. However, it is difficult to apply such a resin using the halogen-based flame retardant to a certain use due to its smoke (caused by combustion) and corrosion. Therefore, in order to solve such problems caused by using the halogen-based flame retardant, research and development on a flame retardant thermoplastic polyurethane resin using a halogen-free flame retardant have been recently conducted.
For example, U.S. Pat. No. 4,413,101 disclosed a thermoplastic polyurethane resin using a high molecular weight resin including polyarylphosphonate and polyarylphosphonatocarbonate as a flame retardant in an amount of 20 to 40 parts by weight, in which an oxygen index is increased thereby increasing flame retardancy. However, herein, physical properties and flaming-drip were not included.
Meanwhile, U.S. Pat. No. 4,542,170 disclosed improvement of flame retardancy and flaming-drip of a thermoplastic polyurethane resin by using pentate salts of amino-s-triazine, and nitrogen containing phosphates (such as amine phosphate, ammonium phosphate, and ammonium polyphosphate), as a flame retardant However, it was found that this method results in significant reduction of a mechanical property such as tensile strength.
Also, U.S. Pat. No. 5,110,850 disclosed a flame retardant thermoplastic polyurethane resin using only melamine as a flame retardant, in which flame retardancy is improved and thus meets the UL (Underwriter's laboratory) rating UL 94VO. However, in this method, flaming-drip was not included.
Therefore, it is required to prepare a flame ret t thermoplastic polyurethane resin capable of improving flaming-drip and flame retardancy and maintaining a mechanical property. Additionally, requirements for a resin having more improved flame retardancy have been increased.